1. Field of the Invention
The present invention relates to ultrasound imaging and, more particularly, to a transmit/receive circuit for an ultrasound imaging device.
2. Description of the Related Art
Ultrasound imaging systems generally operate according to a "pulse-echo method." Such systems must be capable of alternately transmitting and receiving. When transmitting, one or more piezoelectric transducer elements, for example arranged in a linear or two-dimensional array, are excited to high-frequency oscillation by electrical pulses emitted by a transmitter, thereby generating an ultrasound pulse that may be directed at an object to be imaged. This ultrasound pulse is echoed back towards the transducer from some point within the object; for example, at boundary layers between two media with differing acoustic impedances. Then, when receiving, the "echo pulse" is received by the transducer element and converted into a corresponding electrical input signal (i.e., the "echo signal") that is fed to a receiver equipped with sensitive preamplifiers for enhancing the signal. The amplified signal may then be fed to a signal processor for evaluating the echoed image data to generate a visual image.
The transmit pulse and the echo pulse must be separated from one another for the transmit and receive conditions of the ultrasound imaging system to operate effectively. To this end, a transmit/receive circuit is associated with each transducer element in the ultrasound imaging system. Each transmit/receive circuit selectively connects a respective transducer element to either the transmitter or the receiver, depending on whether the transducer element is operating in the transmit mode or the receive mode.
Decoupling the transmitter from the receiver is principally desirable because of the differing amplitudes of the transmitted and received signals. For instance, the transmit pulses used to excite the transducer elements have comparably high voltage amplitudes, typically greater than 100 Volts. On the other hand, the received echo signals are comparatively weak, requiring amplification by sensitive preamplifiers in the receiver. The input stage must therefore electrically decouple the receiver from the transmitter during the transmission to protect the preamplifiers from being damaged by the strong transmit pulses coming from the transmitter. Decoupling the receiver from the transmitter is also desirable during receiving to prevent noise generated by the transmitter from disrupting the signals input to the receiver. Similarly, decoupling the transmitter from the receiver is desirable during transmitting to prevent the receiver from reacting to the transmitter in a manner that might disrupt or distort the shape of the transmit pulse.
Various transmit/receive circuit circuits are known. Such circuits are known to be active (i.e., controlled by a switching signal) or passive (i.e., function automatically). Active transmit/receive circuit circuits are generally realized by various kinds of active devices such as transistors, which vary their properties as a function of the potential or current applied on their control inputs. Such active circuits are disadvantageous in that a separate control line must be provided to each such circuit in the ultrasound device. Obviously, the control signals must be provided by additional control circuitry which must synchronize application of the control signals to the transmit receive cycle. If the synchronization fails, damage to the receivers may result. While passive implementations are known, such passive circuits are generally limited in their dynamic range.
Accordingly, there is a need for a transmit/receiver input stage that is both automatic and has a broad dynamic range. There is a further need for an ultrasound front end circuit employing such a transmit/receive input stage having improved DC isolation and noise suppression characteristics.